The present invention generally relates to node apparatuses and communication networks, and more particularly to a node apparatus which is effectively applicable to a network layer and an OSI physical layer of a local area network, for example, and a communication network having such a node apparatus.
A throughput versus delay time characteristic shown in FIG. 1 is often used to evaluate the performance of a bus type local area network (LAN). This characteristic is a relationship between a transmitted communication quantity (that is, traffic) and a waiting time until the communication is completed. Generally, in the bus type LAN, the delay time is small when the generated traffic is low but the delay time is large when the generated traffic is high because it is necessary to wait until a communication network which is to be used in common becomes available. In FIG. 1, a curve I shows a throughput versus delay time characteristic of a carrier sense multiple access with collision detection (CSMA/CD) system LAN and a curve II shows a throughput versus delay time characteristic of a token ring system LAN. The CSMA/CD system LAN and the token ring system LAN are typical LANs.
For example, the CSMA/CD system LANs such as Ethernet and star-shaped LANs prescribed under IEEE 802.3 have simple protocols, and the communication efficiency is high because the delay time is small when the traffic is low. However, when the traffic is high, many collisions occur and the throughput becomes restricted. This tendency of the restricted throughput becomes notable as the traffic increases, and the throughput eventually decreases when the traffic exceeds a certain limit.
The throughput of the CSMA/CD system LAN becomes restricted as described above for the following reasons. That is, the probability that packet collisions occur becomes high as the traffic becomes high, and the delay time increases due to re-transmissions of packets. In addition, packets which are destroyed by the collisions occupy the communication network meaninglessly and the capacity of the communication network is reduced thereby. Accordingly, when the traffic becomes high and is no longer negligible with respect to the capacity of the communication network, the throughput then decreases. In order to prevent these problems, it is necessary to design the system to run at a data transmission rate considerably higher than an anticipated traffic, but this would require expensive apparatuses and transmission paths.
In the token system LANs such as the token bus system LAN prescribed under IEEE 802.4 and the token ring system LAN prescribed under IEEE 802.5, the increase in the delay time as the traffic increases is small compared to that of the CSMA/CD system LAN and the communication efficiency is high. But the delay time does not become small even when the traffic is low. In addition, the protocol is complex, and the token system LANs require complex and expensive apparatuses. Therefore, it is impossible to employ a high data transmission rate when inexpensive apparatuses are used.
A Japanese Published Patent Application No. 58-40384 discloses a CSMA/CD system in which one communication is completed without damaging the packet even when a collision occurs. Hence, the increase in the delay time is small even when the traffic is high, and the communication efficiency is high. However, the construction of the apparatuses is still complex because the collision is detected by comparing addresses of an output packet and an input packet thereof. Moreover, this system does not satisfy the IEEE 802.3 standards. In communication systems, one important factor is the compatibility with the existing systems, and the product value is poor when the existing standards cannot be satisfied.